The overpressure system and abnormal high-pressure system in petroliferous basins and their relationship with hydrocarbons have long been researched by geologists. In general, the fluid pressure in an open sedimentary formation equals to the hydrostatic pressure at the corresponding burial depth^[1]. The formation fluid pressure in a confined or special geological environment can be lower or higher than the hydrostatic pressure, which is called the abnormal pressure^[1⇓-3]. Abnormal high pressures are formed by various mechanisms such as under-compaction, hydrocarbon generation, tectonic compression, fracturing, diapirism, hydrothermal condition, clay transformation, permeation, and overpressure transmission^[3⇓⇓-6]. Several criteria are currently used for the classification of abnormal pressure. Hunt suggests the pressure coefficients that are less than 0.96 as abnormal low pressure, 0.96 to 1.06 as normal pressure, 1.06 to 1.20 as high-pressure, and greater than 1.20 as abnormal high pressure^[2]. According to the theory of petroleum geology and petroleum exploration, the formation in which the pressure coefficient is less than 0.75 is generally defined as ultra-low-pressure formation, 0.75-0.90 as low-pressure formation, 0.90-1.10 as normal pressure formation, 1.10-1.40 as high-pressure formation, and greater than 1.40 as ultra-high-pressure formation^[3]. China petroleum industry standard defines the gas reservoirs in which the pressure coefficients are less than 0.9 as low-pressure gas reservoir, 0.9 to 1.3 as normal-pressure gas reservoir, 1.3 to 1.8 as high-pressure gas reservoir, and greater than 1.8 as ultra-high-pressure gas reservoir^[7]. In the above classifications and definitions, pressure coefficients, which are the ratio of the formation pressure to the hydrostatic pressure, are used as criteria, but the naming is related to the concept of pressure strength. This naming rule causes confusions: the “high pressure” and “low pressure” above actually refer to pressure coefficients, which are easily misunderstood with the absolute values of fluid pressures. In addition, these classifications are too simple to accurately describe the characteristics of the related overpressure systems. In this research, in order to meet the needs of exploration and exploitation practices, and accurate description of abnormal pressure systems, the pressure coefficient between 0.9 and 1.2 is defined as normal pressure, greater than 1.2 as overpressure, and less than 0.9 as negative pressure. Further, pressure coefficient of 1.2-1.6 is defined as weak overpressure, 1.6-2.0 as strong overpressure, and greater than 2.0 as extremely strong overpressure; pressure coefficient of 0.7-0.9 is defined as weak negative pressure, 0.5-0.7 as strong negative pressure, and less than 0.5 as extremely strong negative pressure. In previous researches, the formation overpressure had been observed and studied in many foreland basins in central and western China, especially in foreland thrust belts and depressions. The overpressure is mainly caused by tectonic compression, hydrocarbon generation, under-compaction, pressure transmission, and overpressure residual. Previous researches around the relationship between overpressure system and hydrocarbon accumulates in foreland basins mainly focused on overpressure sealing. However, these studies concentrated in the overpressure systems in hydrocarbon reservoirs, and few discussions has been made on the macroscopic law of overpressure development, overpressure systems in deep and ultra-deep formations, the overpressure systems in typical foreland basins, and the relationship between overpressure system and giant hydrocarbon fields. In this paper, measured formation pressure data, drilling fluid density in typical wells, and log-calculated pressure profiles were collected. Based on the geological setting characterization of natural gas, the overpressure systems and their cause of development were analyzed, and the relationship between overpressure systems and giant gas accumulations was discussed.

A foreland basin refers to the long and narrow depression belt between the orogen front and the craton block^[8-9]. Structurally, it can be divided into the foreland slope and the foreland uplift, which are composed of the front thrust belt and the front depression of the orogen, as well as the flexure of the craton block edge. Most of the foreland basins in central and western China are formed on the basis of sedimentary blocks instead of craton blocks in tectonically superimposed basins. The evolution of the foreland slope and foreland uplift is mainly controlled by the deformations of the pre-foreland basin deposits; these deformations evolve into orogenic belt later. In the post-foreland basin, the evolution and transformation of the slope and uplift are quite different in each stage^[10]. As a result, the formation mechanism and characteristics of overpressure systems in foreland basins of central and western China are quite different. In this research, the overpressure systems in some typical foreland basins such as West Sichuan foreland basin, Kuqa foreland basin and Zhunnan foreland basin were studied, and the similarity and difference of their characteristics and formation mechanism are discussed in detail.

The West Sichuan foreland basin is a deformed foreland basin, located in the front of Longmen Mountain. It formed on the western margin of the basement of Yangtze Craton in Late Triassic. Mainly marine sediments deposited in the Sinian to early-middle Triassic of pre- foreland basin sedimentary period. In the late Triassic of pre-foreland basin sedimentary period, carbonate gentle slopes, deposits of transition from marine to continental facies, and terrestrial lacustrine basin deposition developed. In the post-foreland basin period, the Jurassic depression lacustrine basin deposition and the Cretaceous-Tertiary intermontane basins deposits developed^[11]. Since 1950s, the industrial gas flow has been obtained in more than 20 strata from the Sinian Dengying Formation to the Lower Cretaceous Jianmenguan Formation^[12], showing a multi-layered, multi-rock type gas bearing system. Both normal pressure gas reservoirs and overpressure gas reservoirs are observed, and the overpressure gas reservoirs are distributed in both the middle-shallow layers and the deep-ultra-deep layers.

Previous studies mainly focused on the overpressure residual, the overpressure development, and its formation mechanism in the Upper Triassic Xujiahe Formation of the Mesozoic^{[13⇓⇓-16]}. However, there is no systematic research on the overpressure systems in deep-ultra-deep layers, or those in the foreland slopes and foreland uplifts. Based on the transformation from drilling fluid density to apparent pressure coefficient, as well as the measured pressure data from formation tests, this study reveals the basic characteristics of overpressure system in West Sichuan foreland basin.

The normal pressure systems are distributed in certain areas that are strongly influenced by Himalayan tectonic movement, such as the middle-shallow layers in the Longmen Mountain front and to the south of Qiongxi- Xiongpo Fault, and the Weiyuan structure and adjacent areas. In other areas, the weak-extremely strong overpressure systems are widespread, showing the heterogeneous planar distribution of the formation pressure which is seriously influenced by faults (Fig. 1).

A multi-cycle overpressure system is observed in foreland depressions of the West Sichuan foreland basin. For example, in the anticlines of the Jiange-Jiulongshan structure, the pressure coefficient profile shows 4 peaks and 5 troughs (Fig. 2): (1) From the Cretaceous-Jurassic formations to Upper Triassic Xujiahe Formation, the pressure coefficient increases from normal pressure and weak overpressure to strong overpressure and extremely strong overpressure; (2) From the Middle Triassic Leikoupo Formation to the Lower Triassic Jialingjiang Formation, the pressure coefficient increases from weak overpressure to strong overpressure; (3) From the Lower Triassic Feixianguan Formation to the Lower Permian Qixia Formation, the pressure coefficient increases from strong overpressure to extremely strong overpressure; (4) From Silurian formations to Cambrian formations, the pressure coefficient increases from strong overpressure to extremely strong overpressure, and reduced to weak overpressure in Sinian.

In the central to northern part of West Sichuan depression and the foreland slope, the overpressure systems are observed vertically in continuous layers with great thickness. For example, in the Shuangyushi-Jiulongshan area, the overpressure layers add up to thousands of meters with different pressure coefficients, while the normal pressure layers are only distributed in the mountain front and some shallow layers that are buried less than 1000 m (Fig. 2). Besides, in the Moxi-Gaoshiti-Longnüsi area, overpressure exists in the Lower Jurassic-Cambrian formation series in the foreland uplift with a total thickness of nearly 5000 m. Normal pressure is only distributed in the deep to ultra-deep Sinian layers and the Jurassic layers that are buried less than 1000 m (Fig. 3).

Overpressure systems in West Sichuan foreland basin are characterized by regional difference (Fig. 2). In the main structures of Jiulongshan and the Jiange area in its southwest slope, multi-cycle overpressure develops in continuous layer series. On the contrary, in the Yuanba area of Jiulongshan southeast slope, normal pressure gas reservoirs develop in the Permian Formations, and separate the thick overpressure layers into two overpressure systems: the medium-deep system and the ultra-deep system. In Shuangyushi area, only two overpressure cycles exist. The upper overpressure cycle is composed of the weak overpressure system in the Middle-Lower Jurassic and to the Xujiahe Formation, the strong overpressure system in the Lower Triassic Jialingjiang Formation, and the weak overpressure system in the bottom of the Middle Permian Formation. The lower overpressure cycle is composed of the weak overpressure system in the Middle Permian Qixia Formation and the strong overpressure system in the Devonian Guanwushan Formation. The vertical overpressure profile near Well Longgang 70 is quite different from that of Jiange area and Shuangyushi area.

In the West Sichuan foreland uplift, the pressure systems are highly heterogeneous, and most of them are monocyclic overpressure systems (Fig. 3). The overpressure system is not observed in Weiyuan structure, while the monocyclic overpressure system with great total thickness develops in Gaoshiti-Moxi-Longnüsi area. The formation pressure coefficient in Weiyuan-Ziyang is below 1.1, and even the Jialingjiang salt-gypsum layer is a normal pressure system. From top to bottom, the pressure coefficient in the thick layers in Gaoshiti-Moxi- Longnüsi area changes from low to high and then back to low, showing typical monocyclic overpressure characteristics.

The complex overpressure system in the West Sichuan foreland basin formed in ancient periods. Wang et al.^[16] suggested that both ancient sedimentary and structural overpressure systems existed in the Xujiahe Formation of the West Sichuan depression. The former was formed by under-compaction and hydrocarbon-generating pressurization, and the latter was formed by tectonic compression pressurization. Through numerical simulation studies, Xu et al.^[17] concluded that the pressure coefficient in the source rocks in Xujiahe Formation could reach up to 2.1 at the peak of hydrocarbon generation. Generally, overpressure systems were considered as the result of a combination of geological processes in previous studies, including tectonic stress, hydrocarbon-generating pressurization, under-compaction, and fluids residual^{[13⇓⇓-16,18 -19]}. In this research, it is concluded that the current large-scale overpressure systems in the West Sichuan basin are formed by two mechanisms: (1) overpressure sealing by plastic gypsum salt layers, and (2) ancient over-pressured fluids residual. In addition, the two overpressure mechanisms happened in different time.

The first formation mechanism of the present overpressure system is the overpressure sealing of the salt-gypsum layer. This formation mechanism mainly occurs in the salt gypsum layers of the Middle Triassic Leikoupo-Jialingjiang Formations and below. It should be noted that the mechanical property of gypsum salt is controlled by temperature and confining pressure. If it is buried less than 3000 m in the West Sichuan basin, the temperature would be lower than 100 °C and the critical confining pressure less than 65 MPa^{[20⇓⇓-23]}. The long-term uplift since late Cretaceous in Sichuan Basin caused exposure and denudation of the covering layers, and the plastic gypsum salt layers in some areas transformed from plastic to brittle, leading to the formation of overpressure gradient belt or normal-pressure belt with brittleness. A typical example is the Jialingjiang-Leikoupo Formation in the central Sichuan foreland uplift. In the Gaoshiti-Moxi-Longnüsi area (Fig. 3) the Jurassic-Cretaceous strata was denuded by about 2000 m influenced by the Himalayan tectonic movement, and the buried depth of the Jialingjiang-Leikoupo Formation decreased from 4000-5000 m in the late Cretaceous to 2500-3500 m at present. As a result, strong-extremely strong overpressure systems (pressure coefficient of 1.8-2.3) develop in the middle-lower Jialingjiang Formation with burial depth greater than 3000 m, and weak-strong overpressure systems (pressure coefficient of 1.4-1.8) develop in the upper Jialingjiang Formation and the Leikoupo Formation. In the Weiyuan Gasfield and adjacent districts, the gypsum salt intervals in the Jialingjiang-Lekoupo Formation are buried less than 2500 m in depth and have totally become brittle. The formation pressure coefficient mostly ranges from 1.0 to 1.1, with no overpressure sealing formed yet. In addition the gypsum salt layers in the West Sichuan depression are buried at the depth of 4500-5500 m, and forming the strong-extremely strong overpressure systems (Fig. 2). However, due to the small difference of the pressure coefficients in these layers and the overlying strata, the position of regional sealing capacity is relatively weakened.

The second formation mechanism of the present overpressure system is the overpressure residual by ancient fluid which is widely observed in the Mesozoic clastic layers in the West Sichuan foreland basin, and related overpressure systems are widely formed in the Xujiahe Formation from the north central Sichuan depression to the west of Guang'an-Hechuan area in the Chuanzhong foreland uplift. The pressure coefficient in the central Sichuan basin is 1.2-1.6, and reaches 1.4-2.3 in the West Sichuan depression. The highest pressure coefficient develops at the Laoguanmiao-Zhebachang-Jiange-Jiulongshan-Yuanba area, mostly ranges from 2.0 to 2.3^[18], showing the characteristics of extremely strong over-pressure. Since the Himalayan tectonic movement, the Jurassic-Cretaceous strata in West Sichuan have been eroded by 2000-4000 m^[24] during the long-term uplift. The normal pressure and weak overpressure systems formed in the middle-shallow layers along with the large-scale fluid pressure unloading, while the fluid overpressure has long been stored in the tight rocks due to poor pressure unloading in middle-deep layers.

In the West Sichuan foreland basin, the formation mechanisms of the overpressure systems in each structural unit are highly different. In the West Sichuan foreland depression, the pressure of the paleo-fluids stored in the Mesozoic clastic layers is very high, forming a regional strong-extremely strong overpressure system. Therefore, the high-pressure paleo-fluid residual in the Mesozoic clastic rocks is the dominant formation mechanism, while the overpressure sealing by the plastic salt gypsum layers of the underlying Jialingjiang-Leikoupo Formation is of secondary importance. On the contrary, in the central Sichuan foreland uplift, the pressure of the paleo-fluids stored in the Mesozoic clastic layers is as low as weak overpressure. The over-pressure fluids residual in clastic rocks therefore acts as the secondary mechanism in the regional overpressure system formation, while the overpressure sealing by the plastic salt gypsum layers of underlaying Jialingjiang-Leikoupo Formation becomes the dominant formation mechanism. In the south part of the West Sichuan depression, overpressure system was not formed due to the widespread development of faults; similarly, the Weiyuan structure and adjacent areas experienced strong deformation during the late Himalayan tectonic movement^[24-25], and overpressure system was either not formed in.

In summary, the overpressure systems in the West Sichuan foreland basin develop in vertical layer series with great thickness, and show great regional differences. In the foreland depression area, multi-cycle overpressure systems develop, while in the foreland uplift area, monocyclic overpressure systems develop, and show great difference in pressure systems. These paleo-fluid overpressure systems are mainly formed by tectonic compression, hydrocarbon-generating pressurization, and under-compaction, while the present fluid overpressure systems are formed by plastic gypsum salt sealing and paleo-fluid overpressure residual. The present overpressure systems are apparently influenced by Himalayan tectonic movement, they can’t formed in the areas strongly deformed during the Himalayan period.

The Kuqa foreland basin is a typical superimposed foreland basin with four evolutionary stages: Triassic period peripheral foreland basin, Jurassic period fault basin, early Cretaceous depression basin, and Cenozoic rejuvenated foreland basin^[26]. Source rocks formed mainly in late Triassic peripheral foreland basin and early-middle Jurassic fault basin, sandstone reservoirs with great thickness formed in early Cretaceous, and regional salt-gypsum caprocks formed in the Cenozoic rejuvenated foreland basin. This source-reservoir-cap association experienced fast burial and hydrocarbon generation, and its kerogen is mainly composed of Ⅲ-type organic matter^[27]. Since oil and gas exploration in Kuqa foreland basin in the 1950s, it is concluded that most of the natural gas is accumulated in the Cretaceous-Tertiary layers, except for the oil-bearing structures in the shallow layers of the Jurassic foreland thrust belt. Most of the natural gas accumulations are overpressure gas reservoirs^[28].

The regional overpressure system in the Kuqa foreland basin is mainly distributed in the gypsum salt layer and underlying layers, which is composed of dual-overpressure systems. According to previous research on the Cretaceous-Tertiary layers in the Kelasu structure and the Triassic-Jurassic layers in Well Yinan 2, it is concluded that the dual overpressure systems developed in Tertiary gypsum salt layers-Cretaceous layers, and Jurassic source rocks-Triassic layers^[29].

In the planar view, the upper overpressure system covers the main part of Kuqa depression, while the normal pressure system is mainly distributed in north piedmont and south slope zone. The distribution of the overpressure system is obviously controlled by structural belts. The strong-extremely strong overpressure belts with pressure coefficient greater than 1.8 are mainly distributed in the Kelasu structure and the east part of the Qiulitage structural belt, showing an obvious influence of linear structures (Fig. 4).

The dual overpressure system shows a superposed distribution vertically in the profiles in the main part of Kuqa depression (Fig. 5). The Cretaceous-Tertiary overpressure system is mainly distributed in the Kelasu-Qiulitage structural belt, and the Triassic-Jurassic overpressure system is mainly distributed from the north part of Kuqa depression to the deep layers in the thrust belt. Over-pressured strata are widely observed in the Kelasu structural belt and the Paleogene subsalt layers to the south. The pressure coefficient of the over-pressured strata not only decreases with the burial depth, but also is obviously controlled by the height of the thrust structures. For example, the Kelasu 2 structure is characterized by strong-extremely strong overpressure, its formation pressure coefficient reaches 2.16 in the bottom of the gypsum salt layer and 2.08 in the top Cretaceous gas reservoir, while decreased to 1.86 with the increase of burial depth. Generally, the pressure coefficient decreases as the distance between the reservoir and the bottom of the gypsum salt layer increases, the pressure coefficient of the Kelasu 2 structure is obviously higher than that of the Keshen structure. The Paleogene Dina gas field is characterized by extremely strong overpressure: the pressure coefficient reaches 2.24 at Well Dina 22, which is located at the top of the structure, and reaches 2.13 at Well Dina 202, which is located at the limb; while at Well Dina 11, which is located at the lower structural position, the pressure coefficient decreases to 2.05; and at Well Dina 102 the pressure coefficient is as low as 2.02. Apparently, the farther away from the top of the anticline structure, the smaller the overpressure intensity is. In the Kuqa piedmont monoclinic belt, the subsalt strata transfer to normal pressure system due to the exposure of the formation and unloading of the fluid pressure gradually. For example, the pressure coefficient in the Paleogene salt gypsum layer at Well Kecan 1 is up to 1.91, while in the Cretaceous sandstone layer it is down to 1.28, and in the Cretaceous sandstone at Well Bashi 2 it decreases to only 1.06. Due to the tight rocks in the Middle-Upper Jurassic layers, the strong overpressure system is developed in the deep Triassic-Jurassic layers. For example, the pressure coefficient in the Jurassic layers at Well Yinan 2 is as high as 1.84, and in the Triassic layers it still reaches 1.74. It is predicted that overpressure system also exists in the coal-bearing source rocks of the north Kuqa depression and the deep layers of the Kelasu structural belt.

Previous studies on the overpressure systems in the Kuqa depression show that the overpressure formation mechanisms include tectonic stress, tectonic emplacement, hydrocarbon-generating pressurization, under- compaction, salt gypsum sealing, overpressure transmission, gas-filling, gas column buoyancy, structural pumping, pressure sealing and fluid thermal pressurization^{[28⇓⇓⇓⇓⇓-34]}. Zhang et al.^[34] suggested that the formation of the Dina 2 gas reservoir is synchronous with the formation of the overpressure systems: the Dina 2 gas reservoir was under normal pressure system in the Kangcun oil and gas infilling period, and evolved into an overpressure system as under-compaction developed in the rapid deposition during Kuqa period; further, from the erosion period of Kuqa Formation to the tectonic compression in the Quaternary, the main overpressure system and gas reservoir were formed. Wang et al.^[30] concluded that sedimentary overpressure systems formed in the Cretaceous strata before the deposition of Kuqa Formation, and since the Kuqa period the sedimentary overpressure systems began to shrink and the tectonic compression overpressure evolved. Based on the study of the formation pressure in the Kuqa foreland basin, this paper concluded that the above viewpoints have limitations. The overpressure system should be studied in separate systems, because the formation mechanisms of the Cretaceous-Paleogene upper overpressure system and the Triassic-Jurassic lower overpressure system are different.

The formation mechanisms of the Cretaceous-Paleogene overpressure system is mainly salt gypsum overpressure sealing and hydrocarbon-generating pressurization. The development of salt gypsum overpressure sealing is the sealing condition for the formation of regional overpressure systems. Both the salt gypsum layers and regional overpressure system are observed from the Kelasu structural belt to the Qiulitage structural belt in the Kuqa depression, as well as the adjacent depression area. However, overpressure system is not observed in other areas (Fig. 4) due to different reasons. Overpressure system was not formed in the north piedmont slope of the Kuqa depression due to the erosion of salt gypsum layers. In the southern slope, on the contrary, the thickness of the salt gypsum layers reaches 100-500 m; but still, the Lower Cretaceous layers are normal pressure systems with the pressure coefficient of 1.08-1.20, even though overpressure systems formed in the Paleogene salt gypsum layers in structures such as Yudong, Yangtake and Yaha with the pressure coefficient of 1.4-1.6. Hydrocarbon-generating pressurization is another necessary mechanism for the formation of regional overpressure in the upper system. Due to the low-degree thermal evolution of the source rocks in the south slope belt (Fig. 5), R_o of the Triassic-Jurassic source rocks is below 1.2% in most cases. The thermal evolution of source rocks in the north area is generally high, where the R_o of Triassic-Jurassic source rocks is greater than 1.2%, and reaches 1.4%-2.0% in most areas. During the Neogene Kuqa deposition period, pressure increased by hydrocarbon-generating reach a high peak^[35].

In addition, the strong-extremely strong overpressure in linear belts is the result of superposition of tectonic movement and salt gypsum overpressure sealing. The formation of strong-extremely strong overpressure systems in compressional structural belt are related to tectonic action, which mainly includes overpressure transmission caused by structural swabbing, gas-filling pressurization and tectonic compressing pressurization^[34].

The formation mechanism of the Triassic-Jurassic lower overpressure system is mainly hydrocarbon-generating pressurization and overpressure residual. Hydrocarbon-generating happened in the deep layers of the Kuqa depression-piedmont thrust belt^[29,35], except for the southern slope, and overpressure residual formed in the deep layers from the north monoclonal belt to the Yiqikelike thrust belt. The amount of surface erosion has reached over 2000 m^[24], as the piedmont thrust belt has been strongly folded and uplifted since the Late Himalayan movement. The strata above the Cretaceous lose the sealing capability due to either the lack of salt gypsum layer or its transformation into brittleness^[36]. On the contrary, the Jurassic rock was well tight, and the porosity of its deep-layer sandstone is as low as 5%. In addition, the lithology of the Middle Jurassic is mainly mudstone, making it has certain sealing capacity for the overpressure system resulted by hydrocarbon generation in deep layers. Moreover, it only experienced short-term uplift and erosion, which can be only 2-3 Ma^[30,35]. As a result, overpressure fluids residual occurred in the Lower Jurassic.

In conclusion, the regional overpressure system in Kuqa foreland basin is composed of dual overpressure systems. The formation mechanisms of the upper regional overpressure system are salt gypsum overpressure sealing and hydrocarbon-generating pressurization. The strong-extremely strong overpressure system formed in linear structural belts is mainly formed by the superposition of tectonic movement and salt gypsum sealing. The formation of the lower overpressure system is the result of hydrocarbon-generating pressurization and overpressure residual.

The southern Junggar foreland basin is located in the central and south part of the Junggar basin, and is further divided into the south piedmont thrust belt, fold anticline belt in southern margin, Changji depression belt, the northern shope belt and the Baijiahai Maqiao uplift belt, etc.^[37]. Southern Junggar foreland basin is a typical foreland basin that was formed by superimposition of multi-type foreland basins and depressed basins, and the thickness of its deposition reaches 15 000 m^[10,38]. Five sets of source rocks are formed in the southern Junggar foreland basin, including the Middle Permian Lucaogou Formation, the Upper Triassic Huangshanjie-Haojiagou Formation, the Middle-Lower Jurassic Badaowan-Toutunhe Formation, the lower Cretaceous Qingshuihe Formation, and the Paleogene Anjihaihe Formation. The major source rocks formed in Permian-Jurassic layers^[39]. Major gas reservoirs are composed of the Badaowan Formation, Toutunhe Formation, Qigu Formation and Kalaza Formation in Upper Jurassic, Qingshuihe Formation and the Paleogene Ziniquanzi Formation^[39]. Regional caprocks are the Anjihaihe Formation, Qingshuihe Formation, and Toutunhe-Qigu Formation^[40]. Previous exploration has already found small anticline oil reservoirs such as Qigu, Dushanzi, Anjihai, Tugulu, and kaindike, oil and gas reservoirs such as Horgos and Gaoquan, and small anticline gas reservoirs such as Hutubi and Mahe^[38⇓-40]. Based on the drilling fluid density and formation pressure testing data, this research discussed the variation law and formation mechanisms of the overpressure systems in the southern Junggar foreland basin.

The overpressure systems in Southern Junggar foreland basin are mainly distributed in the Paleogene, Lower Cretaceous and Middle-Lower Jurassic layers. It was concluded that many overpressure layers are distributed in the Anjihaihe Formation and Badaowan-Sangonghe Formation^[41⇓-43].

In the Jurassic strata of the southern Junggar foreland basin, the overpressure intensity reaches its peak in the fold anticline belt of the south margin, and gradually decreases towards the north direction. It is concluded in previous research that the Neogene and Paleogene overpressure systems were mainly formed in Huo-Ma-Tu fold anticline areas^[41⇓-43]. Recent drilling results revealed that overpressure systems also develop in the Neogene and Paleogene strata of the Gaoquan anticline and Xihu anticline, and the Cretaceous overpressure system extends to some part of the foreland slope belts. The overpressure system is not observed in the piedmont thrust belt, while extreme strong overpressure systems are widely observed in the fold anticline belt on the north side of the thrust belt. The pressure coefficient of these systems is greater than 2.0, and gradually decreases northwards, eastwards, and westwards (Fig. 6).

In profiles, the overpressure systems in the middle- shallow layers are limited and shift quickly, and the overpressure systems in the deep-ultra-deep layers are widely, continuously and stably distributed. The Neogene overpressure systems are formed in relatively thin layers with weak intensity and limited distribution (Fig. 7). Most Neogene reservoirs are normal pressure systems with pressure coefficient of 1.0-1.1, only the reservoir in the Shawan Formation of the Dushanzi Oilfield has a strong overpressure system with pressure coefficient of about 1.73.

The Paleogene overpressure system is characterized by relatively large intensity, good continuity and stability, and thin thickness. The overpressure systems change from the extreme strong overpressure system in the fold anticline belt to the strong overpressure system in northern slope area and weak overpressure system in foreland uplift area (Fig. 8). The overpressure systems were widely formed in the Paleogene strata in the west part of the southern Junggar foreland basin. The Paleogene overpressure system is connected not only with the Neogene and Cretaceous systems in Gaotan 1 well area, but also with the bottom of the Neogene Shawan Formation and Cretaceous layers in Well Xihu 1-Well Ka 6. In the oil and gas reservoirs that have been discovered currently, both overpressure and normal pressure systems are observed in the Paleogene layers. For example, the pressure coefficient of the Huoerguosi reservoir is 2.36- 2.46, and that of the Anjihai oil reservoir is 2.18, while the pressure coefficient in the Hutubi gas reservoir is only 1.03.

The Cretaceous overpressure system is characterized by strong intensity in the south and west, and weak intensity in the north and east. The whole system shows stable and continuous distribution in the Qingshuihe Formation and limited distribution in the Donggou Formation. The south piedmont thrust belt is in a normal pressure system. On the contrary, the Cretaceous strata in Well Gaotan 1 in the footwall of the thrust fault have an extremely strong overpressure system, whose pressure coefficient reaches 2.2-2.3. The Cretaceous strata at Well Xihu 1 are in a weak overpressure system with pressure coefficient of 1.4-1.6, and the extremely strong overpressure system is only observed in the middle sublayers with the pressure coefficient above 2.0. In Well Ka 6, the pressure coefficient decreases to less than 1.4 (Fig. 7). The pressure coefficient in the Cretaceous strata strong overpressure system in Well Tugu 1 of the footwall of thrust fault is greater than 1.80-1.86. The Donggou Formation in Well Dafeng 1 is a normal pressure system with the pressure coefficient of 1.14. Strong overpressure system is only observed in the Qingshuihe Formation with the pressure coefficient of 1.6-2.0. The thin mudstone in the middle of the Qingshuihe Formation in Well Fangcao 1 is a weak overpressure system with pressure coefficient below 1.4. In Well Pencan 2 in the Maqiao Uplift, normal pressure system developed (Fig. 8).

The overpressure systems in the Jurassic and underlying strata are stable, continuous and widely distributed, and show strong intensity in the south and weak intensity in the north. The exploration wells drilled into the Jurassic strata are mainly located in the piedmont thrust belts, foreland slopes and foreland uplifts of the southern Junggar foreland basin, and fewer are located in the depression area. The overpressure intensity reaches its peak in the depression area. For example, the Jurassic stratum in Well Gaotan 1 has an overpressure system with the pressure coefficient above 2.0 (Fig. 7). The Triassic-Jurassic strata in piedmont thrust belt have a normal pressure-weak overpressure system: the pressure coefficient of Qigu structure is 1.21-1.40. The depression area has a strong overpressure system: the pressure coefficient of Upper Jurassic in Well Dafeng 1 is 1.8. The slope area has a strong overpressure system: the pressure coefficient of the Jurassic layers in Well Fangcao 1 is 1.74. The foreland uplift area has a weak overpressure-extremely strong overpressure system, for example, the pressure coefficient of Triassic-Jurassic layers in Well Pencan 2 is 1.5-1.6; to the north, the pressure coefficient of Jurassic-Paleozoic layers in Well Moshen 1 is 1.80-2.12 (Fig. 8). Therefore, the strong overpressure systems are mostly observed in the Jurassic layers of the south Junggar basin, especially the in the Middle Jurassic and underlying strata.

In summary, the overpressure system in the southern Junggar foreland basin was formed in multiple layers. In addition, their scale and intensity increase with the buried depth. In planar view, the overpressure systems are less distributed in the foreland thrust zone (i.e., piedmont zone) and more distributed in the fold anticline zone of the foreland depression. Some of them are observed in the slope zone, while at Maqiao salient of the foreland uplift zone, overpressure system developed only in the Middle Jurassic and underlying strata.

Previous studies have concluded that the formation mechanisms of overpressure systems in the southern Junggar foreland basin mainly include tectonic compression, under-compaction, hydrocarbon generation, dehydration of clay minerals, hydrothermal pressurization, and overpressure compartment^[41⇓-43]. Based on the study of the formation pressure, structural deformation and burial history of the source rocks, this research concluded that the discussion should focus on the formation mechanism of regional overpressure system, instead of that of special overpressure system only.

The regional overpressure system is mainly formed by under-compaction and hydrocarbon-generating pressurization. For example, the Paleogene-Neogene overpressure system is mainly related to mudstones, and most of them have not reached the hydrocarbon generating threshold. As the buried depth increases, the overpressure intensity gets stronger. Tectonic movement has little contribution to the formation of this overpressure system. The tectonic compression is the strongest in the piedmont belt of the south Junggar basin, while there is no overpressure system caused by tectonic compression in the Tertiary strata. In the depression area, the overpressure system caused by the under-compaction of mudstone is widely distributed due to their great burial depth. The extremely strong overpressure system can be formed by the tectonic stress at local compressional fold anticlines, such as the Huoerguosi reservoir with the pressure coefficient of 2.46. The Jurassic-Cretaceous strata are buried under the hydrocarbon generating threshold, and the scale of the overpressure system is positive related to the burial depth.

The formation of thick to ultra-thick overpressure systems is the result of many formation mechanisms. Due to the regional overpressure sealing by the overlying overpressure system, the overpressure systems with great thickness were formed by the superposition of Cretaceous and underlying strata in the depression area, the Lower Cretaceous and underlying strata in the slope area, and the Middle Jurassic and underlying strata in the Maqiao salient area of the foreland uplift. For example, high overpressure coefficient is observed in Well Gaotan 1 continuously from Jurassic to Neogene layers, which was formed by the Neogene and Paleogene under-compaction and the Cretaceous and Jurassic hydrocarbon-generating pressurization. The two effects accumulated and formed overpressure in layers as thick as 4000 m vertically. Another example is the Permian-Triassic hydrocarbon-generating pressurization, which is sealed by the overlying Middle-Lower Jurassic hydrocarbon-generating pressurization in Maqiao salient.

The large-scale overpressure system in the southern Junggar foreland basin is formed in multiple layers, and characterized by limited and variable development in middle-shallow layers, and stable and continuous development in deep-ultra deep layers. The regional overpressure system was mainly formed by the under-compaction and hydrocarbon-generating pressurization. The formation of thick and ultra-thick overpressure system is the result of the accumulation of under-compaction, hydrocarbon generation, and overpressure sealing by overlying overpressure systems.

Since the formation of all foreland basins is under compressive setting, they have structural characteristics such as rapid burial in the front depressions, thrust faults and linear structures in the thrust belts, and stable development in the foreland uplifts. As a result, the overpressure systems in typical foreland basins share common features in distribution and formation mechanisms.

The overpressure systems have the common characteristics of multi-type and multi-layer superimposed development, and their genesis is related to the rapid subsidence filling in the late stage of foreland basin and the transformation of basin types. Thick and superposed overpressure systems were formed in many sets of continental and marine strata in the west Sichuan foreland basin (Fig. 2). Similarly, superposed Triassic-Paleogene overpressure systems were formed in the foreland depression-foreland thrust belt of Kuqa foreland basin (Fig. 5). In the southern Junggar foreland basin, the under-compaction overpressure layers were formed in Cenozoic strata, and the hydrocarbon-generating overpressure layers were formed in Mesozoic strata; and they superposed in a large area in the foreland thrust belt and depression area (Figs. 7 and 8).

The linear developed strong-extremely strong overpressure systems are commonly formed in closed thrust structures, and their formation are related to the strong tectonic compression in the late tectonic stage. The Himalayan tectonic compression in the west Sichuan Basin has a great influence on the formation of these closed overpressure systems, and related pressure increase in the Laoguanmiao-Zhebachang area accounts for 80%-100% of the total overpressure intensity^[16]. The strong-extremely strong overpressure belt was formed under the tectonic compression in Kuqa foreland basin. Similarly, the overpressure system in the south Junggar foreland thrust belt was formed by the Himalayan tectonic compression^[43].

The overpressure systems in foreland basins show strong intensity in foreland depression belts, weak intensity in the area from foreland slope to foreland uplifts, and moderate intensity in deep layers of foreland uplifts. The formation of overpressure system is mainly related to the rapid deep burial and hydrocarbon-generating pressurization in foreland depression area, as well as the overpressure residual in deep layers in foreland uplift. The overpressure strata with giant thickness were formed in the west Sichuan foreland basin with the rapid deposition of the Xujiahe Formation, and the rapid hydrocarbon generation in the middle-late Jurassic (Fig. 2). The hydrocarbon-generating pressurization in Kuqa foreland basin mainly happened in the deep layers from the Kuqa depression belt to the piedmont thrust belt^[29,35]. In the southern Junggar foreland basin, the overpressure system was mainly formed in the anticline belt, and its formation mechanism is mainly related to the under-compaction and rapid hydrocarbon generation caused by rapid burial^[41⇓-43].

It is concluded that the top of overpressure systems or compartments^[2,5] are favorable places for oil and gas accumulation: the overpressure system can be good caprocks^[44-45], and acts as important driving force for oil and gas migration^[41⇓-43]. Large-scale secondary gas reservoirs can be formed by the multi-phase flow of natural gas out of the compartment caused by tectonic movement^[16]. Oil and gas reservoirs can be formed inside, above, or below the overpressure layers^[29,34]. Four kinds of reservoir-cap assemblages under different pressure environments are established in deep-ultra-deep caprocks and large gas fields^[36,46]. According to the classification standard of gas fields in China, the middle-sized gas field contains proved geological reserves of (100-500)×10⁸m³, the large gas field contains proved geological reserves of (500-3000)×10⁸m³, the oversized gas field contains proved geological reserves of (3000-10 000)×10⁸m³, and the giant gas field contains proved geological reserves of more than 10 000×10⁸m^3[47]. It is more meaningful to discuss the relationship between overpressure systems and large gas fields with geological reserves of more than 500×10⁸ m³ as the overpressure systems in foreland basins are mostly formed in deep to ultra-deep layers.

The onshore large-scale gas fields in China are mainly distributed in foreland basin, and closely related to the development of regional overpressure system, such as the Kela 2 gas field, Keshen gas field, Dina gas field and Dabei gas field in the Kuqa foreland thrust belt^[28]; Yuanba and Shuangyushi gas field in the west Sichuan foreland thrust belt, and Anyue gas field and Hechuan gas field in the west Sichuan foreland foreland uplift^[12,48]; and Gaoquan gas field in the south Junggar foreland depression.

The primary condition for large-scale gas accumulations in overpressure systems and underlying strata is the sufficient gas supply. Anyue large gas field with marine carbonate rocks in the west Sichuan foreland uplift contains proven reserves of 11 100×10⁸ m³, and is supplied by the source rocks of the mudstones in the Lower Cambrian Maidiping-Qiongzhusi Formation. The large subsalt gas field in the Kuqa foreland basin contains proven reserves of 14 100×10⁸ m³, and is supplied by the source rocks of Jurassic and Triassic coal-bearing mudstones. Daily oil and gas production of Well Gaotan 1 in the Gaoquan oil and gas reservoir which is supplied by the source rocks of Jurassic to Permian reached 1213×10⁴ m³ and 32.17×10⁴ m³, respectively, which shows a great potential of large-scale oil and gas field in deep layers of the southern Junggar foreland basin^[40].

The development of large-scale reservoir bodies and structural traps is the essential condition for large-scale gas accumulation in overpressure systems and underlying layers. In the Kela 2, Keshen and Dabei gas fields in Kuqa foreland thrust belt, folded anticline structures are frequently observed, and the thickness of the sandstone from Paleogene to Lower Cretaceous Bashijiqike Formation reaches 150-300 m. The thickness of the sandstone reservoirs in the Xujiahe Formation of Jiange and Yuanba in the west Sichuan foreland depression and Anyue, Hechuan, Guangan and Penglai in the foreland uplift reaches 40-260 m. The dolomite reservoir of Longwangmiao Formation in the Anyue large gas field is widely distributed in Moxi-Longnüsi area, and its thickness reaches 50-180 m. The thickness of the dolomite reservoirs in Sinian Dengying Formation reaches 100-450 m^[12].

Stable and complete development of the overpressure system provides the sealing potential for the large-scale gas accumulations in the overpressure system and underlying layers. Gaoshiti-Moxi-Longnüsi area is a long-term inherited paleo-uplift with relatively stable structure. Giant gas accumulations were formed in the deep overpressure system sealed by the overpressure residual system develops from the Mesozoic Xujiahe Formation to Jurassic strata and the plastic salt gypsum overpressure residual system in the Middle-Lower Triassic strata. The proved geological reserve of natural gas reaches more than 13 000×10⁸ m³. On the contrary, the adjacent Weiyuan structure is formed by the folding and uplifting during the Late Himalayan tectonic movement, which lost the two sealing overpressure systems. As a result, a middle-sized gas field with proven geological reserves of about 401×10⁸ m³ was formed in Sinian strata^[24]. In the Gaoquan structure of the southern Junggar foreland basin, faults were formed only in the overpressure system, and large-scale oil and gas accumulation formed in the underlying Cretaceous-Jurassic strata^[40]. On the contrary, in the Tugulu structure, faults cut through the overpressure systems in the Cretaceous-Jurassic strata and Paleogene strata, and only a small-scale oil reservoir was formed in the Paleogene Ziniquanzi Formation. In summary, the stability and integrity of the overpressure system is crucial for the formation of large gas fields.

Previous research concluded that in most cases, oil and gas accumulates at the bottom of the overpressure system or in the overlying normal-pressure strata, and also accumulates inside the overpressure system with less stability, but the research in this paper concluded otherwise.

Controlled by migration channels, large gas fields often develop in the middle and upper parts of the foreland thrust fault systems in the overpressure system. The large gas fields such as Kela 2 and Keshen in Kuqa foreland thrust belt formed at the top of the faults cutting through the source-reservoir system in the overpressure system (Fig. 5). The Shuangyushi gas reservoir in the west Sichuan foreland thrust belt formed in the Qixia-Maokou Formation in the upper-middle fault systems that cut through the lower part of overpressure system (Fig. 2). Gaoquan reservoirs in the foreland thrust belt at the west of the southern Junggar foreland basin formed in the Jurassic-Cretaceous strata at the top of fault system that cut through the middle-lower part of the overpressure system (Fig. 7).

Controlled by the overpressure sealing, large gas fields often form in the overpressure system, as well as the natural pressure areas and relative low-pressure areas at the bottom of overpressure system. Both the sub-layer 2# and 4# of the Sinian Dengying Formation in the ultra-large Anyue gas field in the west Sichuan foreland uplift are normal pressure reservoirs, and the pressure coefficients ranges from 1.07 to 1.10, while the pressure coefficients of the overlying overpressure layers with giant thickness are mostly 1.4-2.2 (Fig. 3). The pressure coefficients of the deep-ultra deep Permian dolomite Shuangyushi gas reservoir and Permian-Triassic reef beach gas reservoir in Yuanba are obviously lower than those of upper and lower strata in overpressure system of Western Sichuan foreland (Fig. 2). The pressure coefficient of the gas reservoir in Qixia Formation is 1.36-1.37, in the overlying Maokou Formation is 1.8, and in the underlaying Devonian is 1.48-1.65.

Controlled by the source-reservoir assemblage, large gas fields in overpressure system mostly formed in large- scale reservoirs near source rocks. The Sinian gas reservoir in the Anyue gas field formed under the high-quality thick source rocks in the Lower Cambrian Qiongzhusi Formation, and the Lower Cambrian Longwangmiao gas reservoir formed above the source rocks. In the central of the Sichuan basin, the source rocks formed in the upper Xujiahe overpressure system. Large-scale Anyue tight gas reservoir with pressure coefficient of 1.4-1.6 formed in the second member of Xujiahe Formation, with proved geological reserves of more than 2000×10⁸ m³. The Longwangmiao reservoir in giant overpressure system in central Sichuan basin is mainly distributed in Moxi area. In other layers, only middle-small gas accumulations formed due to the small-scale reservoirs.

Although several large-large gas fields have been discovered in typical onshore foreland basins in China, this research suggests that there are still many new fields. For example, the marine carbonates in the overpressure residual system in the uplift and slope zone of the west Sichuan foreland basin, and the Mesozoic tight sandstone gas in the north-central foreland depression; the Cretaceous in the frontal triangle of the thrust belt and the Jurassic-Triassic tight sandstones in the foreland depression of Kuqa foreland basin; and the deep-ultra-deep layers in the southern Junggar foreland basin.

The regional overpressure systems in typical foreland basins in the central and West China developed with different formation mechanisms. The present regional overpressure system in the west Sichuan foreland basin is mainly controlled by the Himalayan tectonic movement and overpressure residual; it formed in thick and continuous layers, and is characterized by regional differences. The regional overpressure system in the Kuqa foreland basin is mainly controlled by plastic salt gypsum sealing and hydrocarbon-generating pressurization. It is composed of the superposition of dual overpressure systems in salt gypsum layers and underlying strata. The regional large-scale overpressure system in the southern Junggar foreland basin is mainly controlled by under-compaction and hydrocarbon-generating pressurization; it developed in multiple layers and is characterized by limited and changeable development in middle-shallow layers, and stable and continuous development in the deep-ultra-deep layers. In summary, these overpressure systems shared common characteristics: they were formed in multiple types and layers; strong overpressure system generally develops in foreland depressions, the strong-extremely strong overpressure system generally develops in linear structure of closed foreland thrust belt, and the strong-extremely strong overpressure generally develops in deep layers of foreland uplift area.

Three conditions are crucial for the formation of large- scale gas fields in overpressure system and under it: large- scale source rocks, large-scale reservoir bodies, and stable and complete regional overpressure system. Large-scale gas fields often develop in 3 places: (1) the middle and upper parts of fault system in the overpressure system; (2) normal-pressure and relative low-pressure areas in or at the bottom of the overpressure system; (3) large-scale reservoirs that develop near source rocks. The large scale deep-ultra-deep gas fields can be formed by the regional residual and sealing of the overpressure fluid.

Large-scale gas accumulations may develop in many new areas, such as the marine carbonates in the sealing overpressure system in the uplift and slope zones of the west Sichuan foreland basin, and the Mesozoic tight sandstone gas in the north-central foreland depression; the Cretaceous in the frontal triangle of the trust belt and the Jurassic-Triassic tight sandstones in the foreland depression of Kuqa foreland basin; and the deep-ultra- deep layers in the southern Junggar foreland basin.